The light source used in a fluorescence microscope, Raman spectrometer or similar optical measurement device is required to focus a laser beam into the smallest possible spot in order to improve the measurement accuracy. However, when the laser beam is simply focused by a focus lens, the spot size of the laser beam cannot be smaller than the diffraction limitation which is determined by the wavelength of the laser beam and the numerical aperture of the focus lens.
Non-Patent Document 1 discloses a laser beam suitable for reducing the spot size. FIG. 21A schematically shows a cross section of this laser beam at a plane perpendicular to the beam (this cross section hereinafter will be simply referred to as the “section of the laser beam”). The shaded portion in this figure corresponds to the area where the beam is present. The thick arrows in the figure indicate the polarizing direction. The laser beam has a ring-shaped cross section having no intensity in the central region and is polarized in the direction from its center toward the outer area (i.e. in the radial direction). A laser beam having such a cross-sectional shape and polarization is hereinafter referred to as the “radially-polarized ring-shaped laser beam.” According to Non-Patent Document 1, the radially-polarized ring-shaped laser beam 21 can be focused into a laser beam whose spot size is smaller than the diffraction limitation. Conventionally, such a small spot could only be obtained over an extremely short range of up to approximately one wavelength in the direction of the optical axis. By using the new laser beam, it is possible to obtain a small spot over a range equal to or even longer than ten times the wavelengths (see Non-Patent Document 2).
As shown in FIG. 21B, when the radially-polarized ring-shaped laser beam 21 is focused by a lens 23, the electric-field components oscillating in the directions perpendicular to the laser beam (in the x and y directions) at the condensing point 22 are cancelled out, while the electric-field component oscillating in the direction parallel to the laser beam (in the z direction) remains without being cancelled out. As a result, at the condensing point 22, an oscillation of the electric field in the direction parallel to the travelling direction of the beam is created over a wide range of approximately ten times the wavelengths on the beam axis (see Non-Patent Document 2), Such a polarization (i.e. z-polarization) does not occur on the beam axis when a normal beam is used. Using the z-polarization is advantageous in some cases; for example, in a Raman scattering measurement, it is possible to observe a scattering that does not occur when a polarization perpendicular to the laser beam is used (see Non-Patent Document 3).